Wet-strength paper and method of producing same



rates Unite The present invention is concerned with wet-strength papers and a new and improved process for the production of papers having improved wet-strengths.

In accordance with the invention, it has been found that the direct impregnation of certain papers in a sub stantially dry state and under certain conditions with aqueous dispersions of water-insoluble emulsion copolymers of monoethylenically unsaturated monomers containing about 3 to 10% by weight of units of a member selected from the group consisting of maleic acid, aconitic acid, and salts and anhydrides thereof serves to produce an unexpected and surprising increase in Wet-strength when the amount of copolymer deposited is in the range of 050% to 32%, and preferably 2 to 8%, based on the weight of the fiber, and the impregnated paper is heated at a temperature of 240 F. to 450 F. for a period of about 30 minutes at the lower temperature to about 2 to minutes at the higher temperature. This heating may be the drying step or it may follow the drying of the impregnated paper. The salts are those of monovalent metals, such as the alkali metals, ammonia, and watersoluble mono-amines, such as diethylamine, triethylamine, diethanolamine, triethanolamine, and morpholine. The ammonium salts are preferred because of the volatility of the ammonia on drying of the impregnated papers and low cost. Since maleic acid and aconitic acid are polycarboxylic acids, neutralization with ammonia and amines can produce salts of partial amides, such as the salt of maleamic acid. Such salts are included herein.

The types of papers with which the present process is concerned are those having a basis weight between 10 and 65 lbs. for a 500 sheet ream in which the sheets are 24" x 36". The cellulose fibers (wood pulps) from which the papers are made must also be highly beaten so that they have Canadian freeness values in the range of 260 to 455 cc. They may be sized papers and particularly those sized with rosin in conventional manner using alums such. as the conventional papermakers alum. The amount of rosin sizing may be that which is customary, namely from 0.2% to 2% by weight of the rosin based on the weight of fibers in the paper.

When the process of the present invention is applied to papers comprising substantial amounts of artificial fibers, such as those formed of regenerated cellulose rayon fibers, cellulose acetate, nylon, polyethylene, polyethylene glycol terephthalate (Dacron), vinyl or acrylic resin fibers, such as polymers of acrylonitrile, especially those having at least 75% acrylonitrile in the polymer, copolymers of vinyl chloride and vinyl acetate, copolymers of vinylide-ne chloride with vinyl chloride, or acrylonitrile, such as saran, and the like, the increase in Wet strength is also remarkable especially at the preferred low levels of application of the maleic or aconitic acid copolymers or salts previously indicated. Such papers may comprise, for example, from 5% to 95% of one or more of the artificial fibers mentioned in admixture with 95% to 5% respectively of well-beaten wood pulp fibers (Canadian freeness between 260 and 455 cc.).

The papers obtained by the process of the present invention are adapted when finished to be employed as wrapping papers (when prepared from sized stock) or as tissue papers including toilet tissue, toweling, creped or atent 3,017,291 Patented Jan. 16, 1962 ice not, diapers, and for other moisture-absorbent uses (when prepared from unsized stock). The impregnated papers of the present invention are also adapted to be subjected to subsequent treatments with aqueous media, as in saturation or impregnation with aqueous dispersions or suspensions of other gums, resins, pigments, fillers, and the like, since-the products of the present invention have excellent wet strength which helps them to withstand the forces normally encountered during such wet treatments.

The copolymer is largely formed of polymerized units of acrylonitrile or esters of acrylic or methacrylic acid. For example, the polymer may contain 90 to 97% by weight of an ester of acrylic acid with a lower alcohol having from 1 to 4 carbon atoms, such as methyl acrylate, ethyl acrylate, or butyl acrylate. Besides the lower alkyl esters of acrylic acid (that is, instead thereof or in addition thereto), the copolymer may contain units of higher alcohol esters of acrylic acid such as cyclohexyl acrylate, n-hexyl acrylate, amyl acrylate, 2-ethyl-hexyl acrylate, t-octyl acrylate, decyl acrylate, dodecyl acrylate, hexadecyl acrylate, or octadecyl acrylate. Besides the ester of acrylic acid, the copolymer may contain units of acrylonitrile or of lower alkyl esters of methacrylic acid, such as methyl methacrylate, ethyl methacrylate, or butyl methacrylate, or even higher alkyl esters of methacrylic acid such as the cyclohexyl and other alkyl methacrylates corresponding to the higher alkyl esters of acrylic acid mentioned hereinabove. The copolymer may also comprise up to 60% by weight of vinyltoluene (the ortho, meta, or para isomers or any mixture thereof), the balance being made up of the acid (or salt) units and acrylonitrile or acrylic acid or methacrylic acid ester units.

The copolymer may have any molecular weight from about ten thousand up to as high as about ten million. The copolymers preferably have molecular weights of about 500,000 (viscosity average) up to as high as several million. In order to produce such high molecular weight polymers, the emulsion procedure is employed, the emulsifier being used in a proportion of about /2% to 6% based on the weight of monomers (total). Conventional aqueous emulsion polymerization techniques are employed. Water-soluble initiators may be used such as the alkali metal or ammonium persulfates in amounts from 0.10% to 3% on the weight of the monomers. Preferably, a redox system is used in which the persulfate or like initiator is used in conjunction with a reducing agent such as sodium metabisulfite in about the same proportion as the initiator. A promoter, such as small proportions of metal ions such as copper, iron, or the like, may be used. The polymerization may be effected as a batch system or by continuous addition of the monomers to the aqueous solution containing initiator, reducing agent, and promoter. Generally, it is unnecessary to heat the polymerization system, and in many cases it may be desirable to cool the system to control the temperature and maintain it between room temperature and C. Agitation during the polymerization is generally desirable but may be omitted. The amount of water may be selected at the beginning of the polymerization procedure with reference to the amount of comonomers to be copolymerized so that any desired concentration on the order of 25% to 50% or more of copolymer is obtained in dispersed condition in the reaction vessel.

It copolymers of lower molecular weight are desired, there is used in the polymerization medium about 0.2 to 5% of a chain-transfer agent, such as a compound of the following group: long-chain alkyl mercaptans, e.g., t-dodecyl mercaptan, isopropanol, isobutanol, long-chain alcohols, e.g., laurylalcohol, t0ctyl alcohol, CCI C Cl and CBrCl The copolymer dispersion may be applied in the acid or salt form at a pH in the range of about 2 to about 11.

Preferably, a neutral or alkaline pH between about 7 and 8.5 is used as this has the advantage of causing less corrosion of the equipment used for application to the paper.

The copolymer dispersion is originally prepared or diluted to a concentration of about 0.5% to 32%, preferably about 2 to by weight of the copolymer for application. The aqueous dispersion of the copolymer is applied to the paper by simply dipping or immersing the paper therein while the dispersion is at a temperature of 70 to 140 F. Advantageously, the paper may proceed continuously through such aqueous copolymer dispersion. For example, it may be passed through such a dispersion in a size press immediately following the normal paper drying drums. Of course, the paper proceeds from the size box into a final drier. When unsized papers pass through the copolymer dispersion, there is approximately 125% wet pickup; Whereas when a sized paper is employed, the wet pickup is generally about to 50% cold or up to 80% if heated so that the concentrationof the copolymer dispersion employed with sized papers should be at least about 2 to 4%.

The paper may be formed from a wide variety of materials. Thus, it may be a kraft pulp, rag pulp, soda, sulfate, groundwood, sulfite pulp, or alpha pulp. Besides employing the normal wood pulp fibers, the paper may be formed of cotton linters and, if desired, the wood or cotton fibers may be used in admixture with fibers from other sources such as jute, hemp, sisal, rags, strings, chopped canvas, and so on.

The finished paper products may be employedas filter papers, disposable diaper fillers, toilet tissue, paper toweling, bibs, and various wrapping materials.

One of the advantages of the procedure of the present invention is the fact that the normal drying equipment employed in conventional papermaking machinery is adequate for drying the paper after impregnation with the copolymer dispersion. Hence, the process can be employed as an on-machine size press coating operation. However, if desired, the paper may be treated in an offmachine converting process by passing the paper through asaturating bath (size tub) and then dried and/ or creped on a. drum drier or other drying equipment. When the copolymers are prepared with relatively little emulsifier present and it is applied in the acid form or in the form ofv an ammonium or volatile amine salt, the application of the copolymer actually serves not only to improve wetstrength but also sizes the paper when unsized paper is the initial starting material. In other words, the copoly mer, especially when in the form of an ammonium or volatile amine salt, after drying down to the insoluble form on the impregnated sheet reduces the penetration of water and/or ink. Most of the wet-strength resins conventionally employed require either a high temperature cure (in excess of the temperature available in the con ventional paper-drying machinery) or require ageing for as much as one month in order to develop a full wetstrength obtainable therefrom. Such wet-strength resins of thermosetting aminoplast type also need a low pH on the acid side to facilitate the cure or development of the wet-strength on ageing. In contrast with this, the copolymer dispersions of the present invention can be applied at a pH of about 7 to 10 where less likelihood of corrosion on the machinery is encountered.

Another advantage of the copolymer dispersions of the present invention is the fact that they need not be applied at higher temperature than 140 F. or even above 70 F. (normal room temperature) to provide good impregnation and pickup, whereas the customary thermosetting aminoplast resins generally have to be at a temperature of 180 F. at the point of impregnation in order to provide efficient pickup by the paper. Also, the copolymer dispersions are of relatively low viscosity so that thorough, uniform impregnation of the paper is readily obtained.

In general, the copolymers of the present invention provide an improvement in wet-strength immediately on drying which is from 50 to greater than the wet-strength obtained with conventionally employed thermosetting aminoplast resins even after 28 days of ageing required for the substantially full development of wet-strength with such aminoplast resins.

The conventional thermosetting aminoplast resins are applied at about 14% solids at about C. at an acid pH of about 4.5 with alum or hydrochloric acid. They have a tendency to gel at this temperature and must be promptly used. Batchesof as much as 200 gallons are made up at a time and any left over after a batch wetstrength paper run must be thrown out. In contrast with this, the aqueous copolymer, dispersions of'the present invention are stable and can be stored indefinitely so that they can be saved for the next wet-strength batch con. version operation.

Typical thermosetting aminoplast types of. wet-strength resins when applied to sized papers either reduce or destroy the sizing effects. The compositions of the present invention either have no efiect on thesize of the sized paper or, when a low proportion of emulsifier is employed for producing the copolymer dispersion, the sizing is increased.

In the following examples which are illustrative of the present invention, the parts and percentages are by weight unless otherwise noted. The tensile strengths (both dry and wet) are in pounds per inch width of the paper as determined by a conventional Scott IP4 tester. The wet strengths were measured. after a one-hour soak in water at room temperature. The percentwet strengthisthe ratio, expressed as a percentage, of the wet strength to the dry strength.

EXAMPLE 1 An aqueous dispersion containing 0.5% of a copolymer of 9.0% by weight of ethyl acrylate and 10% of aconitic acid was applied at 70. F. to a bleached sulfite paper obtained from a pulp having a Canadian freeness of 450 ml. by passing the paper through the dispersion and then through rubber squeeze rolls. The impregnated paper picked up 0.5% of copolymer (on fiber) and was dried at.300 F. for 10 minutes. Similarly a solution of the same copolymer was applied at 2.5%, 5%, and 10% concentration providing 2.3%, 4.6%, and 8.9%v of copolymer on the fiber weight of the dried sheet. of the dispersions were in the range 2.53.2.' The results are given in the following Table I:

Table I W et Tensile, Lbs/In.

Width Wet Tensile Percent of Dry Tensile EXAMPLE 2 Well-beatenbleached sulfite pulp (Canadian freeness of 380 cc.) was made into paper sheets on the Standard Noble-Wood equipment. The water-leaf sheets were then subjected to saturation by the technique of Example 1 (immersion and squeeze) with copolymers of maleic anhydride as indicated below, used as dilute dispersions in water. The sheets were dried at 180 F, then heated at 300 F. for 10 minutes before conditioning and testing. In the table, EA represents ethyl acrylate and M'alA represents maleic anhydride. In runs 4 and 7, the pH was adjusted with ammonium hydroxide.

The pH.

3| Table II Table V Copolymer Tensile pH of Per- Dry Wet Percent Per- Per- PerpH of Per- Strength Percent No. Discent in Tensile Tensile Wet N 0. cent cent cent Dlscent in Wet Per- Perpersion Paper Strength Strength Strength 5 MalA AcA persion Paper Strength cent cent Dry Wet EA MalA no saturant H2O 0.0 42.4 2.2 5 1- 95 5 2. 2 4. 8 21.4 4. 5 21 only 2- 95 5 2. 8 1.0 21. 9 3. 7 17 90 10 8. 9 2.1 39. 4.0 10 3"--- 90 10 2.0 4. 8 21.1 7.0 33 90 10 9. 0 5. 1 44. 3 6. 3 14 4 90 10 9.0 4. 8 23. 2 7. 3 31 95 5 2. 9 0.5 34. 9 3.1 9 5 90 2. 8 0.46 20.4 2. 7 13 95 5 2. 2 5.0 36. 4 4. 7 13 6 90 10 2. 5 1.0 20. 2 4. 5 22 95 5 8. 9 5. 3 37.6 4. 8 13 7 9O 10 9.0 1.0 20.9 4. 5 22 95 5 3. 2 0.5 37. 5 3. 3 9 5-.." Nosaturantzwater only- 0.0 21.4 1.3 6 95 5 2.6 5.0 40.4 5.7 14

EXAMPLE 3 15 EXAMPLE 6 Unbleached kraft pulp was beaten to a freeness of 375 cc. (Canadian) and formed into paper sheets on the Standard Noble-Wood equipment. The sheets were then saturated by the technique of Example 1 with aqueous dispersions (about 3% to 6% concentrations) of the polymers listed in Table III following which they were dried at 200 F., then heated at 300 F. for 10 minutes before conditioning and testing. In the table, EA represents ethyl acrylate, AcA represents aconitic acid, and MalA represents maleic anhydride. The pH values of the dispersions were adjusted with ammonium hydroxide, except in run number 2.

A mixture of equal parts of polyethylene glycol terephthalate fibers (Dacron: 3 denier per filament, A inch) and bleached sulfite fibers (455 cc. Canadian freeness) was formed into paper sheets by the method described in Example 3 above. The Dacron fibers were not subjected to any heating but simply mixed with the beaten bleached sulfite pulp before sheet formation. The papers were saturated by the technique of Example 1 with aqueous dispersions (about l% to 5.5% concentrations) of the polymers listed in Table IV in which BA represents butyl acrylate, then dried and heated as in Example 3.

Bleached kraft pulp beaten to a Canadian freeness of 270 cc. was formed into 60 lb. basis weight (3000 sq. feet) paper sheets, treated by the technique of Example 1 with aqueous dispersions (about 2.5% to 5.5% concentrations) of the polymer saturants of Table V, dried, and heated as in Example 3.

The fiber of Example 5 (270 cc. Canadian freeness) was formed into sheets of 40 lb. basis weight (3000 sq. feet) and treated by the technique of Example 1 with the aqueous dispersions (about 1% to 12% concentrations) of the polymer saturants of Table VI. This series illustrates the effect of pH of the dispersion. The pH adjustments were made with ammonium hydroxide. It is clearly shown that the levels of wet strength realized by this process are substantially unaffected by pH over the range 2.4 to 8.9. Drying and heating were carried out exactly as in Example 3.

Table VI Tensile Per- PerpH: of Per- Strength Percent No. cent cent Discent in Wet BA AcA persion Paper Strength Dry Wet 1 no saturant: 0.0 25. 4 1.1 4

11 0 only.

It is to be understood that changes and variations may be made without departing from the spirit and scope of the invention as defined by the appended claims.

We claim:

1. A method for producing a Wet-strength paper which comprises impregnating throughout its mass a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" by 36" with an aqueous dispersion containing 2 to 32% by weight of a water-insoluble copolymer of 3 to 10% by weight of units of a member selected from the group consisting of maleic acid, aconitic acid, and salts and anhydrides thereof, and at least one other comonomer selected from the group consisting of acrylonitrile, vinyltolnene, esters of acrylic acid, and esters of methacrylic acid, said dispersion being of low viscosity, having a total solids content not over about 36% by weight, and the amount of copolymer therein being at least about by weight of the total solids content of the dispersion, drying the impregnated paper and heating the impregnated paper at a temperature of 240 F. to 450 F. for a period ranging from about 2 to 5 minutes at the higher temperature to about 30 minutes at the lower temperature.

2. A method for producing a wet-strength paper which comprises impregnating throughout its mass a substantially dry paper prepared from cellulose fibers beaten to a Canadian freencss between 260 and 455 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" x 36" with an aqueous dispersion containing 2 to 15% by weight of an ammonium salt of a copolymer of ethyl acrylate and 3 to 10% by weight of aconitic acid, said dispersion being of low viscosity,

having a total solids content not over about 17% by weight, and the amount of copolymer therein being at least about 85% by weight of the total solids content of the dispersion, and heating the impregnated paper at a temperature of 240 F. to 450 F. for a period ranging from about 2 to minutes at the higher temperature to about 30 minutes at the lower temperature.

3. A method for producing a wet-strength paper which comprises impregnating throughout its mass a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and having a basis weight of to 65 lbs. per ream of 500 sheets having a size of 24 X 36" with an aqueous dispersion containing 2 to by weight of a copolymer of a mixture of ethyl acrylate and 3 to 10% by weight of maleic anhydride, said dispersion being of low viscosity, having a total solids content not over about 17% by weight, and the amount of copolymer therein being at least about 85% by weight of the total solids content of the dispersion, and heating the impregnated paper at a temperature of 240 F. to 450 F. for a period ranging from about 2 to 5 minutes at the higher temperature to about 30 :minutes at the lower temperature.

4. A method for producing a wet-strength paper which comprises impregnating throughout its mass a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" X 36" with an aqueous dispersion containing 2 to 15% by weight of an ammonium salt of a copolymer of a mixture of ethyl acrylate and 3 to 10% of maleic anhydride, said dispersion being of low viscosity having a total solids content not over about 17% by Weight, and the amount of copolymer therein being at least about 85% by weight of the total solids content of the dispersion, and heating the impregnated paper at a temperature of 240 to 450 F. for a period ranging from about 2 to 5 minutes at the higher temperature to about 30 minutes at the lower temperature.

5. A method for producing a wet-strength paper which comprises impregnating throughout its mass a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 45 5 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" X 36 with an aqueous dispersion containing 2 to 15% by weight of an ammonium salt of a. copolymer of butyl acrylate and 3 to 10% by weight of aconitic acid, said dispersion being of low viscosity, having a total solids content not over about 17% by weight, and the amount of copolymer therein being at least about 85% by weight of the total solids content of the dispersion, and heating the impregnated paper at a temperature of 240 F. to 450 F. for a period ranging from about 2 to 5 minutes at the higher temperature to about 30 minutes at the lower temperature.

6. A method for producing a wet-strength paper which comprises impregnating throughout its mass a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" x 36" with an aqueous dispersion containing 2 to 15 by weight of a copolymer of a mixture of ethyl acrylate and 3 to 10% by weight of aconitic acid, said dispersion being of low viscosity, having a total solids content not over about 17% by weight, and the amount of copolymer therein being at least about 85% by weight of the total solids content of the dispersion, and heating the impregnated paper at a temperature of 240 F. to 450 F. for a period ranging from about 2 to 5 minutes at the higher temperature to about 30 minutes at the lower temperature.

7. A method for producing a wet-strength paper which comprises impregnating throughout its mass a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" x 36" with an aqueous dispersion containing 2 to 15% by weight of a copolymer of butyl acrylate and 3 to 10% by weight of aconitic acid, said dispersion being of low viscosity, having a total solids content not over about 17% by weight, and the amount of copolymer therein being at least about by weight of the total solids content of the dispersion, and heating the impregnated paper at a temperature of 240 F. to 450 F. for a period ranging from about 2 to 5 minutes at the higher temperature to about 30 minutes at the lower temperature.

8. A method for producing a wet-strength paper which comprises impregnating throughout its mass a substantially dry paper prepared from a mixture of fibers consisting of 5 to by Weight of cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and 95 to 5% by weight respectively of artificial fibers, said paper having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24 by 36" with an aqueous dispersion containing 2 to 32% by weight of a water-insoluble copolymer of 3 to 10% by weight of units of a member selected from the group consisting of maleic acid, aconitic acid, and salts and anhydrides thereof, and at least one other comonomer selected from the group consisting of acrylonitrile, vinyltoluene, esters of acrylic acid, and esters of methacrylic acid, said dispersion being of low viscosity, having a total solids content not over about 36% by weight, and the amount of copolymer therein being at least about 85 by weight of the total solids content of the dispersion, drying the impregnated paper and heating the impregnated paper at a temperature of 240 F. to 450 F. for a period ranging from about 2 to 5 minutes at the higher temperature to about 30 minutes at the lower temperature.

9. A method for producing a wet-strength paper which comprises impregnating throughout its mass a substantially dry paper prepared from a mixture of fibers consisting of 5 to 95% by weight of cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and 95 to 5% by weight respectively of artificial fibers selected from the group consisting of regenerated cellulose, cellulose acetate, nylon, polyethylene glycol terephthalate, vinyl resin, and acrylic resin fibers, said paper having a basis Weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" by 36" with an aqueous dispersion containing 2 to 32% by weight of a water-insoluble copolymer of 3 to 10% by Weight of units of a member selected from the group consisting of maleic acid, aconitic acid, and salts and anhydrides thereof, and at least one other comonomer selected from the group consisting of acrylonitrile, vinyltoluene, esters of acrylic acid, and esters of methacrylic acid, said dispersion being of low viscosity, having a total solids content not over about 36% by weight and the amount of copolymer therein being at least about 85% by weight of the total solids content of the dispersion, drying the impregnated paper and heating the impregnated paper at a temperature of 240 F. to 450 F. for a period ranging from about 2 to 5 minutes at the higher temperature to about 30 minutes at the lower temperature.

10. As an article of manufacture, a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" x 36" impregnated throughout its mass with 0.5 to 32% by weight, based on the dry weight of the fibers, of a dry, insoluble material consisting essentially of a copolymer of 3 to 10% by weight of units of a member selected from the group consisting of maleic acid, aconitic acid, and salts and anhydrides thereof, and at least one other comonomer selected from the group consisting of acrylonitrile, vinyltoluene, esters of acrylic acid, and esters of methacrylic acid.

11. As an article of manufacture, a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and having a basis weight of to 65 lbs. per ream of 500 sheets having a size of 24" x 36" impregnated throughout its mass with 0.5 to 32% by weight, based on the dry weight of the fibers, of a dry, insoluble material consisting essentially of a copolymer of ethyl acrylate with 3 to 10% by weight of aconitic acid.

12. As an article of manufacture, a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 2 x 36" impregnated throughout its mass with 0.5 to 32% by weight, based on the dry weight of the fibers, of a dry, insoluble material consisting essentially of a copolymer of ethyl acrylate with 3 to 10% by weight of maleic anhydride.

'13. As an article of manufacture, a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" x 36" impregnated throughout its mass with 0.5 to 32% by weight, based on the dry weight of the fibers, of a dry, insoluble material consisting essentially of an ammonium salt of a copolymer of ethyl acrylate with 3 to 10% by weight of aconitic acid.

14. As an article of manufacture, a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 and 455 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" x 36" impregnated throughout its mass with 0.5 to 32% by weight, based on the dry weight of the fibers, of a dry, insoluble material consisting essentially of an ammonium salt of a copolymer of ethyl acrylate with 3 to 10% by weight of maleic anhydride.

15. As an article of manufacture, a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 to 455 cc. and having a basis weight of 10 to lbs. per ream of 500 sheets having a size of 24" x 36 impregnated throughout its mass with 0.5 to 32% by weight, based on the dry weight of the fibers, of a dry, insoluble material consisting essentially of an ammonium salt of a copolymer of butyl acrylate with 3 to 10% by weight of aconitic acid.

:16. As an article of manufacture, a substantially dry paper prepared from cellulose fibers beaten to a Canadian freeness between 260 to 455 cc. and having a basis weight of 10 to 65 lbs. per ream of 500 sheets having a size of 24" x 36" impregnated throughout its mass with 0.5 to 32% by weight, based on the dry weight of the fibers, of a dry, insoluble material consisting essentially of a copolymer of butyl acrylate with 3 to 10% by weight of aconitic acid.

References Cited in the file of this patent UNITED STATES PATENTS 1,981,102 Hagedorn et a1. Nov. 20, 1934 2,499,653 Kropa et a1. Mar. 7, 1950 2,544,691 Kugler et a1 Mar. 13, 1951 2,667,462 Wildish et al. Jan. 26, 1954 2,676,934 Butler Apr. 27, 1954 2,790,736 McLaughlin et a1 Apr. 30, 1957 

1. A METHOD FOR PRODUCING A WET-STRENGTH PAPER WHICH COMPRISES IMPREGNATING THHROUGHOUT ITS MASS A SUBSTANTIALLY DRY PAPER PREPARED FROM CELLULOSE FIBERS BEATEN TO A CANADIAN FREENESS BETWEEN 260 AND 455 CC. AND HAVING A BASIS WEIGHT OF 10 TO 65 LBS. PER REAM OF 500 SHEETS HAVING A SIZE OF 24" BY 36" WITH AN AQUEOUS DISPERSION CONTAINING 2 TO 32% BY WEIGHT OF A WATER-INSOLUBLE COPOLYMER OF 3 TO 10% BY WEIGHT OF UNITS OF A MEMBER SELECTED FROM THE GROUP CONSISTING OF MALEIC ACID, ACONITIC ACID, AND SALTS AND ANHYDRIDES THEREOF, AND AT LEAST ONE OTHER COMONOMER SELECTED FROM THE GROUP CONSISTING OF ACRYLONITRILE, VINYLTOLUENE, ESTERS OFACRYLIC ACID, AND ESTERS OF METHACRYLIC ACID, SAID DISPERSION BEING OF LOW VISCOSITY, HAVING A TOTAL SOLIDS CONTENT NOT OVER ABOUT 36% BY WEIGHT, AND THE AMOUNT OF COPOLYMER THEREIN BEING AT LEAST ABOUT 85% BY WEIGHT OF THE TOTAL SOLIDS CONTENT OF THE DISPERSION, DRYING THE IMPREGNATED PAPER AND HEATING THE IMPREGNATED PAPER AT A TEMPERATURE OF 240*F. TO 450* F. FOR A PERIOD RANGING FROM ABOUT 2 TO 5 MINUTES AT THE HIGHER TEMPERATURE TO ABOUT 30 MINUTES AT THE LOWER TEMPERATURE. 